Thermoset polymers form the matrix in filled plastics and fiber-reinforced composites used in many different products. Thermosets are used extensively as adhesives, molding compounds and surface coatings. Three stages are typically used in the processing of thermoset polymers. In the A-stage the resin is still soluble and fusible. In the B-stage, thermosets are nearly insoluble but remain thermoplastic. Though the B-stage material exists in a molten state, this material is relatively short-lived owing to the fact that the temperature used to promote flow also causes the material to crosslink. The C-stage represents the final stage of polymerization, wherein the polymer undergoes crosslinking under the controlled influence of heat and pressure over time. The resultant thermosets build their final structure during this processing, forming a three-dimensional internal structural network of highly-crosslinked polymer chains. The final thermoset material is insoluble and not thermally reformable.
Many composite structures are composed of fibers and thermoset polymers that are generally epoxies. Because of the present restrictions on fiber orientations, the composites containing such thermosets typically deform matrix by dilatation when a mechanical load is applied. Since dilation is an elastic response to the applied load, these composites display low strength, high weight, and/or other limited mechanical performance attributes. Thus, composite materials that include fiber orientations coupled with thermosets have improved distortional properties when subjected to loads. U.S. Pat. No. 7,985,808 to Christensen et al. describes thermosets having a distortional matrix intended for use in composites with the proper fiber orientation, which is herein incorporated by reference in its entirety. A variety of engineering components and structures rely upon distortional thermosets afforded by certain epoxy resin composites.
All epoxies deform by either dilation and/or distortion. Dilation is controlled through non-bonded forces at the molecular level within the epoxy structure and is generally similar among various epoxies. Yet the distortional attributes of epoxies can be highly varied depending upon the chemical formulation of epoxies.
The matrix of epoxy composites will either dilate or distort depending on the nature of the loading and, most importantly, the fiber orientation. The fiber orientations of prepreg materials will force the matrix material to dilate. New design concepts for materials having optimal fiber orientation are exploring the benefits of distorting the matrix as a means for improved performance for these materials. Such design considerations require new epoxy matrix materials having optimized distortion attributes.
Yet distortional thermosets are affected by a variety of environmental elements that can compromise their reliability in epoxy-based engineering components and structures. Polymer or material changes caused by the service environment may lead to premature failure. Examples of such polymer changes and service life conditions include: thermo-oxidation; photo-oxidation (for example, from sunlight/UV-light); cyclic fatigue (for example, from vibration); physical aging (for example, densification); erosion; environmental stress-cracking resistance (ESCR); and sorption of water, fluids, etc. The effect of different service life conditions may be synergistic, leading to surprisingly quick failures. Polymer performance in these areas may be related to both the degree of crosslinking and the chemical nature or polarity of the amorphous polymer. For example, sorption of a fluid by the cured epoxy may lead to chemical changes as well as mechanical changes. For fully cured epoxies, fluid sorption has led to failures, often interrelated, from: swelling, modulus loss, strength loss, stress cracking (ESCR), weight gain, gloss loss, hardness loss, adhesion loss and coloration. Such service environment effects also affect distortional thermosets, where the desired mechanical performance attributes of these materials can be compromised.